5388se is not a necessary prerequisite because, e.g., RSH (R = Me, Et, Ph; pK, = 6.5-10.6)23 removes a IL-OXO group from M 0 2 0 4 ~ +while , phenol (pK, = 10.0)23does not. Of comparable importance, then, is the nucleophilicity of sulfur and the subsequent binding of additional ligands. Further, mercaptans easily remove terminal oxo groups, producing non-oxo molybdenum(V) monomers from M0@42+, Le., stripping the molybdenum completely of both bridging and terminal oxo groups. All the successful reactions described above involve -SH groups; if such groups are omitted, the molybdenum complex is recovered unchanged. The aqueous chemistry of molybdenum, particularly in its higher oxidation states, is dominated by oxo-containing species produced from water, but its nonaqueous chemistry may well be equally dominated by the mercaptan, with the formation of sulfur-coordinated species and the occurrence of molybdenum-catalyzed reactions of mercaptans with various substrate^.^^ As the Mo-containing sites of enzymes may be hydrophobic in nature, these studies reveal a host of interesting possibilities for their modes of reaction.
Acknowledgment. W e thank J. L. Corbin, G. R. Seely, E. I. Stiefel, and G. D. Watt for helpful discussions and Climax Molybdenum Co. for generous gifts of compounds. References and Notes (1) W. E. Newton, J. L. Corbin, D. C. Bravard, J. E. Searles, and J. W. McDonald, Inorg. Chem., 13, 1100 (1974): W. E. Newton, J. L. Corbin, and J. W. McDonald, Proceedings of the International Symposium on N2 Fixation, W. E. Newton and C. J. Nyman, Ed., Washington State University Press, 1976, p 53. (2) G. N. Schrauzer, J. Less-Common Met., 36,475 (1974); Angew. Chem., Int. Ed. Engl., 14, 514 (1975). (3) P. W. Schneider, D. C. Bravard, J. W. McDonald, and W. E. Newton, J. Am. Chem. SOC.,94, 6640 (1972). (4) Throughout this paper, the repeating unit in the dimeric compounds is enclosed in square brackets with the bridging atoms or groups following. (5) All compounds synthesized in this work have satisfactory elemental analysis and are produced in 70-100% yields (except where otherwise stated). Ir, NMR, and/or EPR also support the formulations shown. All reactions go to completion within 5 h. ( 6 ) [MoOL']ZOS has also been prepared from the reaction of MoOzLfZ(L' = SpCN-n-Prp) with P4S10,but not with HzS in the presence of air,' In contrast, we isolate only OMO(SZ)L"Zfrom MoO~L"2(L" = S2CNMe2)and P4S10 anaerobically in benzene while Weiss et al. produce OMo(S2)L'2 from M o O ~ Land ' ~ H2S in acetone under air.7 (7) J. Dirand. L. Ricard, and R. Weiss, Inorg. Nucl. Chem. Left.. 11,661 (1975): Inorg. Chim. Acta, 18, L21 (1976). (8) Those compounds containing terminal molybdenum-oxygen bonds have the two typical' infrared absorptions at -965 cm-', which are replaced in [MoSL]& by a similar doublet at -540 cm-'. v(Mo0Mo) occurs at -730 cm-' in the p-Oz complex' and at -710 cm-' in the N-OS complex. No absorption occurs in this region in the p-Sz analogue. (9) T. Sakurai, H. Okabe, and H. Isoyama, Bull. Jpn. Pet. Inst., 13, 243 (1971): B. Spivak, Z. Dori, and E. I. Stiefel, Inorg. Nucl. Chem. Lett., 11, 501
(1975). (10) W. E. Newton, J. L. Corbin, and J. W. McDonald, Abstracts, 167th National Meeting of the American Chemical Society, Los Angeles, Calif., April 1974, iNOR 35. ( 1 1) If only the oxo group were bridging, then a linear MoOMo system (as in [MoOL2]20) is most likely, which should absorb at -510 nm. These compounds are yellow-orange and do not absorb between 450 and 700 nm. Further, their ir Mo=O absorptions are typical of nonlinearly bridged molybdenum(\/) dimers,',12e.g., [ M O O L ] ~but ~ ~are , lowered by -40 cm-' to -940 cm-'. The weak ir band at -750 cm-' is reminiscent of a single p a x 0 group. These data suggest a bent MoOMo system, probably imposed by other bridging groups. The mercaptides occupy inequivalent positions (two NMR.signais are observed at 35 OC in CDCi,), which also supports a bridging function. The p2-OS and p2-SN(CH3)products exhibit similar ir properties. (12) W. E. Newton, J. L. Corbin, and J. W. McDonald, J. Chem. SOC., Dalton Trans., 1044 (1974). (13) J. I . Gelder, J. H. Enemark, G. Wolterman, D. A. Boston, and G. P. Haight, J. Am. Chem. SOC.,97, 1616 (1975). (14) Yields of -70% are obtained in this reaction for L = S2P(i-C3H7)2. (15) MoL{o-C~H~SN(CH~)]~ is obtained in 5 0 % yield from the filtrate after removal of precipitated [MOOL]~O[ o-C6H4SN(CH3)]. (16) i. F. Gainu!in,'N. S. Garif'yanov, and V. V. Trachevskii. Izv. Akad. Nauk SSSR, Ser. Khim., 2176 (1969). (17) The complexes M o L ( o - C ~ H ~ Xare S ) ~too insoluble for molecular weight determinations, but their formulation is supported by their EPR spectra." They show no ir absorption assignable to v(Mo=O) between 900 and 1000 cm-'. With X = NH, v(NH) is observed at 3290 cm-', which shifts to 2410 cm-' on deuteration. (18) N. Pariyadath, W. E. Newton, and E. I. Stiefel, J. Am. Chem. SOC.,the following paper in this issue.
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(19) R. Banal, C. Bocard, I. %r& de Roch, and L. Sajus, Tetrahedron Left., 1693 (1972). (20) W. E. Newton, D. C. Bravard, and J. W. McDonald, Inorg. Nucl. Chem. Left,, 11, 553 (1975). (21) J. Dirand, L. Ricard, and R. Weiss, J. Chem. SOC.,Dalton Trans., 278 (1976). (22) E. I. Stiefel and J. K. Gardner, J. Less-Common Met., 36, 521 (1974). (23) M. R. Crampton in "The Chemistry of the Thiol Group, Part l", S. Patai, Ed., Wiley, New York, N.Y., 1974, pp 396-410. (24) J. W. McDonald, J. L. Corbin, and W. E. Newton, Inorg. Chem., in press, and unpublished results.
W. E. Newton,* Grace J.-J. Chen, John W. McDonald Contribution No. 567 Charles F. Kettering Research Laboratory Yellow Springs, Ohio 45387 Received April 6, 1976
Monomeric Molybdenum(V)Complexes Showing Hydrogen-1, Hydrogen-2, and Nitrogen-14 Superhyperfine Splitting in Their Electron Paramagnetic Resonance Spectra. Implications for Molybdenum Enzymes Sir:
An electron paramagnetic resonance (EPR) signal characteristic of molybdenum(V) has been found in all molybdoenzymes which catalyze substrate Under certain conditions, near isotropic superhyperfine splitting from a single proton is clearly resolved. These results are particularly significant because the hydrogen responsible for this splitting in xanthine oxidase has been shown to originate in the sub~ t r a t eEvidence .~ indicates that this proton is transferred from substrate to enzyme in conjunction with the flow of two electrons in the same d i r e c t i ~ n . Mechanistic ~?~ schemes based on these results have been suggested for xanthine ~ x i d a s e and ~-~ other molybdoen~ymes.~-~ Alternate suggestions favor either the location of the exchangeable proton on an N atom ligated to Mo6,7or the formation of a molybdenum h ~ d r i d eEvalu.~ ation of the feasibility of these suggestions requires EPR analysis of simple Mo(V) complexes wherein the magnitude of proton and nitrogen superhyperfine couplings can be assessed. Although proton couplings have previously been reported for Mo comp1exes,*-l0 these have been for species in s o l u t i ~ nor~ ,for ~ ill-defined reaction productsI0 and no isolated and stoichiometrically distinct compounds which display this property have been studied. This communication describes our results in which both proton and nitrogen superhyperfine splittings have been resblved for isolated Mo(V) compounds. The magnitude of the coupling constants and the observed proton exchangeability are consistent with previous proposals made for M o enzyme^.^-^ In addition, significant temperature effects on the spectral shape are reported. The monomeric Mo(V) complexes presented here are prepared by the general procedure of Newton et al." Their EPR spectra (Table I) strongly support the formulas shown. As an example, the spectrum of 1 is discussed. The spectrum, as
(Et2NCS2)Mo 1
displayed in Figure 1A, shows hyperfine splitting from the I = %, 95Moand 97Monuclei (as satellite spectra) although only the central pattern from even M o isotopes ( I = 0) is shown. Superhyperfine splitting is clearly resolved for two equivalent N and two equivalent H atoms confirming both the state of ligation of the complex and the state of protonation of the li-
August 18, 1976
5389 Table I. EPR Parameters for Some Monomeric Mo(V) Complexes
Complex
Solvent
(g)f 0.001
( A ) 9 5 , 9 7 MG~ ,
( A )L, G (L)
Mo(S2CNEt2)(HNSC6H4)2 (1)
CHC13
1.990
38.0 f 0.5
Mo(S2CNEt2)(CH3NSC6H4)2 (2) Mo(S2CNEt2)(DNSC6H4)2 (3)
CHC13 CH2C12
1.985 1.989
38.0 f 0.5 38.0 f 0.5
2.4 f 0 . 1 ( 1 4 ~ ) ; 7.4 f O.I('H) 2.8 f 0.1 (I4N) 2.5 f 0.1 (I4N); 2.0 f 0.1 (2H)
MO(S~CNE~~)(%~H~)Z MOO[ SIP(i-Pr)z](OSC6H4) M o O [ S ~ P ( ~ - P ~ ) ~ ] ( O+ S Cpy~ H ~ ) Xanthine oxidase
CHC13 CH2C12 CH2Cl2
2.000 1.971 1.976 1.974
31.8 f 0.5 39.2 f 1 37.2 f 1 a
38 f 1 (3'P) 28.6 f 1 (31P) 12 ('H)
( A ) for 95Mocan be estimated as 34 G for the so-called very rapid signal of xanthine oxidase (R. C. Bray and J. C. Swann, Sfrucf.Bonding (Berlin), 11, 107 (1972)).
biological and inorganic systems, we urge strong caution in correlating g and A values with structure. Both g and A (but especially g) depend not only on the donor atom set but also quite strongly on both coordination number and detailed coordination geometry.I4 For example, the eight coordinate complex { M o [ S ~ C N ( C ~ H ~ also ) ~ ] shows ~ ) + g = 1.979 and A = 37 GI3 but is clearly very different from the complexes discussed here and from the M o site in enzymes. All signals reported herein show temperature dependence with a minimum line width occurring somewhere above the freezing point of the solvent. For the N-methyl complex, 2, the broad room temperature spectrum (Figure 1C) is replaced by a sharper spectrum (Figure 1D) a t -40 OC. Further cooling (still above the freezing point) rebroadens the spectrum and, in the frozen glass, the lines are broad (at least down to 100 K) Figure 1. (A) Room temperature EPR spectrum of Mo(S2CNEt2) and the hyperfine splittings characteristic of the solution are (HNSC6H4)2; (B) computer simulation of the above; (C, D, and E) EPR not res01ved.I~Thus, to attain highest resolution and hence spectra of Mo(S2CNEtz)(CH3NSC6H4)2 at room temperature, -40 and maximum information from solution spectra, variable tem-90 'C, respectively. perature studies are imperative. It seems highly probable that, as many EPR studies are reported at room temperature and/or gand nitrogens. Spectral simulation (Figure 1B) gives a H = liquid N2 temperature where the solutions are frozen, much 7.4 and a N = 2.4 G. These values differ dramatically from inherent information is lost. W e strongly recommend that (for those assigned p r e v i o u ~ l yfor ~ ~similar ~ species prepared only Mo(V) and other doublet states) variable temperature studies in solution.I2 To confirm these assignments, Et2NCS2Mo[operformed when increased resolution of hyperfine C6H4SN(CH3)]2 (2) and E ~ ~ N C S ~ M O [ O - C ~ H(3) ~ S N Dbe] routinely ~ or superhyperfine structure is desired.I6 were prepared. The spectrum of the N-methyl complex, 2, The implications of this work for Mo enzymes are clear. shows I4N splitting of 2.8 G in a clear five-line spectrum at -40 While these compounds in no way represent models for the O C in CH2Cl2 (Figure 1D). The N-deuterio complex, 3, shows active site of enzymes, they do illustrate that Mo(V) complexes a more complex pattern for which simulation reveals a N = 2.5 with coordinated N-H groups display H superhyperfine couf 0.2 Gauss and a D = 2.0 f 0.2 G consistent with the reduced pling of the same order of magnitude as that found in the M o magnetic moment of the deuteron compared to the proton. 3 oxidases. Furthermore, N superhyperfine coupling is of sufis prepared from Mo204(S2CNEt2)2 similarly to 1 by using ficiently small magnitude to preclude its resolution in enzymes C6H4ND2SD (v(N-H) 3290 vs. v(N-D) 2410 cm-I). Howshould Mo-N indeed be present.lg Our current results thus ever, when the N-deuterio complex is dissolved in CHC13 for establish the feasibility (if not the veracity) of recent proposals E P R evaluation, the spectrum of 1 is observed, apparently as concerning the mechanism of action of M o enzymes. a result of H-D exchange with C ~ H S O H present as a preservative in the solvent; a reaction which must occur within the Acknowledgment. W e thank Grace J.-J. Chen and Dudley few minutes required to take the spectrum. The N-D complex Bravard for excellent technical assistance in compound syncan be maintained by dissolution in CH2C12 previously equilthesis, Andrea Burns for computational assistance, and Climax ibrated with D20. These results attest to the exchangeability Molybdenum Co. for generous gifts of chemicals. of the proton in question. References and Notes The tabulated g and A(95,97Mo)values reveal some interesting trends. All the non-oxo complexes contain a t least three R. C. Bray in "The Enzymes", 3d ed,P. D. Boyer, Ed., Academic Press, New coordinated S atoms and display g values somewhat higher York, N.Y., 1975. E. I. Stiefel, Prog. Inorg. Chem., in press. than those typically found in xanthine oxidase. In general, the R. C. Bray and P. F. Knowles. Proc. R. Soc. London, Ser. A, 302, 351 larger the number of S ligands, the higher the g and lower the (1968). D. Edmondson, D. P. Ballou, A. van Heuvelen, G. Palmer, and V. Massey. A(95,97Mo)values found. The oxo complex MoO[S2PJ. Bid. Chem.. 248, 6135 (1973). ( i - P r ) ~(OSC&), ] although also possessing three coordinated J. S.Olson, D.P. Bailou, G. Palmer, and V. Massey, J. Biol. Chem., 249, S atoms, displays a lower g value which places both its g and 4363 (1 974). E. I. Stiefel, Proc. Natl. Acad. Sci. U.S.A., 70, 988(1973). A value in the range found for the M o oxidases. The effect of E. I. Stiefel and J. K. Gardner in Proceedings of the First International adding pyridine to this nominally pentacoordinate species is Conference on Chemistry and Uses of Molybdenum, P. C. H. Mitchell, Ed., Climax Molybdenum Co.. London, 1974: J. Less-Common Met., 36, 521 to slightly raise g and lower A . An analogous effect involving (1974). substrate binding may produce the similar g and A shifts obL. N. Duglav and 2 . I. Usmanov, Zh. Strukt. Khim., 16, 312 (1975). served in xanthine oxidase. Despite these similarities between I. F. Gainulin, N. S.Garifyanov, and V. V. Trachevskii, Izv. Akad. Nauk SSSR, ~
Communications to the Editor
5390 Ser. Khim., 10,2176 (1969). (IO) P. C. H. Mitchell and R. D. Scarle, ref 7. (11)W. E. Newton, G. J.J. Chen, and J. W. McDonald, J. Am. Chem. SOC., previous paper in this issue. (12) The spectra obtained by Gainulin et ai.’ for species in solution are very similar to those reported in the present study but the assigned coupling constants differ vastly. For compound 1, the assignment of Gainulin et al. is aN = 5.0 f 0.2and aH = 2.5 f 0.2.Placing these values in our simulation program leads to a spectral pattern similar to that found experimentally. Mathematically either assignment fits the observed spectrum. Upon K methyl substitution, Gainulin et al. note that aN decreases to 2.8 G. We confirm this experimental result with isolated complexes but, in view of our assignment of aN, we note that N-CH3 substitution increases aN by a small amount. This is certainly a more reasonable result than nearly doubling the constant when H is substituted by CH3. Proof for our assignment comes from studies on the N-D system discussed in the text. (13)A. Nieuwpoort, ref 7. (14) See, for example, W. L. Kwik and E. I. Stiefel, Inorg. Chem., 12, 2337 (1973). (15) Frozen glass spectra reveal the complexes to have rhombic or axial g tensors with overall g spread of less than 0.03.Unfortunately, neither ’H nor 14Nsuperhyperfine splittings have been clearly resolved in the frozen glass spectra, the analysis of which will be reported in detail in the full paper. (16) Russian worker^^^^,'^ seem to be acutely aware of these temperature effects and often report highly resolved EPR spectra at odd and seemingly arbitrary temperatures. It was from scrutiny of their experimental results that it first occurred to us to vary the temperature of samples. (17)I. N. Marov. V. K. Belyaeva, A. N. Ermakov, and Y. N. Dubrov. Russ. J. / m g . Chem., 14, 1391 (1969). (18)In support of this argument, we note that 2H splitting is not resolved in Mo enzymes when *H is substituted for lH. The estimated 2H splitting is of the same magnitude as ’% splittings found in Mo(V) complexes2 and therefore the lack of resolution of 14N splitting in Mo enzymes is not at all unexpected.
Narayanankutty Pariyadath, W. E. Newton, Edward 1. Stiefel* Contribution No. 571 Charles F. Kettering Research Laboratory Yellow Springs, Ohio 45387 Received April 6. 1976
Table I. Atomic Coordinates0 Coordinates Atom
104 v
104 x 1949 (1) 969 (4) 1183 ( 5 ) 493 (6) -475 (6) -729 ( 5 ) - 5 (4) 2133 (3) 752 (4) -1174 (4) -1663 (3) -2829 (3) 2498 (7) 2821 ( 5 ) 3472 ( 5 ) 3815 (7) 1784 (10) 2584 (63) 3641 (59) 4149 (86)
2500 1660 (3) 1340 (3) 745 (4) 460 (4) 750 (4) 1343 (3) 1598 (2) 450 (3) -106 (2) 462 (2) 1601 (2) 2500 1760 (4) 1764 ( 5 ) 2500 2500 1323 (40) 1208 (38) 2500
a Estimated standard deviations are given in parentheses for least significant digit.
M atom
+ RfX
-
RrMX
We report here the production of a stable R2Co compound utilizing the metal atom technique. This compound is formed in the reaction of F-bromobenzene with cobalt atoms, and is the first example of an R2M compound produced by the method.
Sir:
Simple RMX and R2M compounds of transition metals such as Fe, Co, Ni, Pd, or Pt have been of interest to quite a number of workers.’ We have found that the “metal atom technique” (metal vapors condensed with organic compounds) has been
c 6 F ~ B r4- Co
-
(C6F5)2CO -t CoBr2 1
Bis(F-phenyl)cobalt(II) ( l ) ,previously reported by Smith and
F\
/ 10
H
Figure 1. The structure of [ $ - C ~ H & H ~ ( C ~ F & C OThe ] . crystallographic mirror plane contains Co, C7, CIO,C, 1, and Hlo.
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-74 ( 1 ) 778 ( 5 ) 2090 (6) 2732 (6) 2051 (7) 129 (7) 138 (6) 2824 (3) 4022 (4) 2668 ( 5 ) 39 ( 5 ) -1181 (4) -2271 (8) -1605 (7) -354 (7) 260 (1 1) -3611 (9) -1919 (74) 99 (66) 1093 (100)
quite useful for preparation of some of the simple nonligand stabilized R M X compounds in the absence of complicating donor solvent^.^ Due to the extraordinary robustness of the Rr-M bond,4 the C F J and C6F5Br reactants have been quite useful to us in terms of production of stable, characterizable R M X compounds.
Metal Atom Synthesis of $-Toluenebis( F-phenyl)cobalt(II) [06-C6H5CH3(C6F~)zco], a New Type of ?r-AreneComplex. X-Ray Structure Determination
H
104 z
August 18, 1976
